1. Field of the Invention
The present invention relates to a new hydrometallurgical process and apparatus for treating foundry sludge and cell bleed electrolyte to recover magnesium as magnesium chloride, while selectively removing calcium and sodium. The present invention also relates to removing calcium and sodium impurities from brines of magnesium chloride.
2. Related Art
In the production of pure magnesium metal, various sludge are generated from the foundry and the electrolytic cell. For example, pure magnesium metal may be cast in a casting furnace, and magnesium alloys may be cast in an alloying furnace. Both of these furnaces will produce residues called sludge which contain mainly magnesium metal, magnesium chloride, sodium chloride, calcium chloride, calcium oxide, and magnesium oxide. This electrolytic cell also produces cell bleed electrolyte containing magnesium metal and various other compounds.
The disposal of these sludge (containing magnesium metal with chlorides and oxide compounds) presents a problem. If stockpiled, this material constitutes a safety hazard since it is reactive and readily emits hydrogen and ammonia gas. The current practice for magnesium producers is to store this material in sealed containers or to landfill the material. This creates a severe environmental issue.
Known methods exist for producing magnesium chloride from mixtures of magnesium and calcium chloride or to separate calcium and sodium from magnesium chloride solution. For example, U.S. Pat. No. 3,516,785 (incorporated herein by reference) describes a process for the recovery of magnesium chloride from sodium, potassium, magnesium chloride and sulphate containing mixed salt solutions, by successive concentrations to precipitate first sodium, and then potassium-magnesium double salts, and disulphate the mother liquor with calcium chloride. More specifically, in the first step, the solution is subjected to solar evaporation to precipitate sodium chloride followed by kainite (KCl.MgSO4.3H2O) with additional NaCl in the second step. The liquor is then desulphated by the use of calcium chloride to precipitate calcium sulphate. However, this patent is more particularly concerned with the selective recovery of magnesium chloride from naturally occurring brines such as found in the oceans and salt lakes.
U.S. Pat. No. 4,100,254 (incorporated herein by reference) describes an industrial process for preparing high-purity magnesia from impure magnesium-containing starting material, wherein the starting material is dissolved in HCl and the resultant acidic solution is subjected to a multi-step treatment for precipitating impurities out of the solution. In this process, sulphate ions are added, for instance, in the form of magnesium sulphate or sulphuric acid to the concentrated solution to convert the calcium ions dissolved in the solution into calcium sulphate which is precipitated and filtered from the solution. However, calcium sulphate has a relatively high solubility, and it cannot air stripped to a very low level.
U.S. Pat. No. 4,341,752 (incorporated herein by reference) describes a method for producing purified and concentrated MgCl2 brine by evaporation and crystallization from brines containing MgCl2, KCl, NaCl and MgSO4, involving recirculation of carnallite and part of the final product brine. The method results in a pure end product by a simple process comprising only one evaporation step and without any addition of chemicals. In both examples presented in this patent, the brine composition assayed 450-455 g/l MgCl2, 14-15 g/L MgSO4, 5-6 g/l NaCl and 2 g/l KCl after evaporation and at 30xc2x0 C. There are two major disadvantages with this method:
The presence of MgSO4 in the brine is unacceptable, particularly for the electrolytic cells, and
This method also does not remove calcium from the brine. If the cell electrolyte is bled to control the calcium in the circuit, it must be removed.
Thus, what is needed is a new technology that has the potential of treating all of the sludge, therefore minimizing the magnesium and chloride losses while selectively removing calcium and sodium to produce a relatively concentrated magnesium chloride solution.
The present invention represents a new hydrometallurgical process and apparatus for recovering magnesium as magnesium chloride solution from various foundry sludge and cell bleed electrolyte while removing calcium and sodium.
According to one aspect of the present invention, recovering magnesium from a magnesium-containing solution includes structure and/or steps for: (i) dissolving, in a water slurry, soluble chloride compounds in the magnesium-containing solution; (ii) acidifying the water slurry to between substantially pH 4 and substantially pH 6; (iii) further acidifying the water slurry to between substantially pH 1 and substantially pH 0, and providing a magnesium chloride solution; (iv) precipitating calcium from the magnesium chloride solution, (v) separating solids from the magnesium chloride solution; (vi) stripping SO2 from the magnesium chloride solution, and (vii) precipitating NaCl from the magnesium chloride solution to provide a concentrated magnesium solution. Note that the process can begin at step (iv) if the magnesium chloride solution is provided.
According to another aspect of the present invention, a method for recovering magnesium chloride from a magnesium-containing sludge, includes the steps of: (i) reducing the sludge preferably to between xe2x88x9210 to xe2x88x9248 mesh; (ii) slurrying the reduced sludge in water to dissolve soluble chloride compounds in the sludge; (iii) acidifying the sludge slurry to between substantially pH 4 and substantially pH 0 by the addition of HCl acid, keeping the slurry potential above substantially xe2x88x92850 mV; (iv) further acidifying the sludge slurry to between substantially pH 1 and substantially pH 6 by the addition of HCl acid until the slurry potential reaches a positive value, and providing a leach slurry; (v) neutralizing the leach slurry to pH 5-6 by the addition of MgO; (vi) sparging the neutralized leach slurry with sulphur dioxide gas to precipitate calcium from the leach slurry; (vii) separating solids from the leach slurry to recover a magnesium chloride solution; (viii) stripping SO2 from the magnesium chloride solution by adding HCl to adjust the pH preferably between pH 0 to pH 3, and sparging with air; (ix) increasing the total chloride concentration of the magnesium chloride solution by one of (a) prills dissolution and (b) HCl sparging, to precipitate NaCl; and (x) separating the precipitated NaCl from the magnesium chloride solution to provide a concentrated magnesium solution.